The transition to high school contains major social stressors and coincides with a sharp increase in depression and other internalizing symptoms. Unfortunately, it has been difficult to identify effective universal preventative interventions for depression and other internalizing disorders among high school students. The proposed research integrates theories and methods across social, developmental, clinical, and biological psychology to address this gap in knowledge. This research focuses on the underlying beliefs or implicit theories that affect reactions to stressful social environments. An entity theory of personality is the belief that people do not have the potential to change. It is known to lead to maladaptive reactions in the wake of social stressors, such as the conclusion that one is a loser or not likable. By contrast, an incremental theory is the belief that people have the potential to change. It has been found to facilitate more hopeful interpretations that social stressors can improve. Preliminary published research with multiple independent samples (N =599) has found that an efficient, but well-timed and highly-persuasive, self-administered intervention to teach adolescents an incremental theory of personality can prevent increases in depressive symptoms over the first year of high school by 40%, nine months post-intervention. However, it is important to estimate the size of this effect in large (N 2,000), heterogeneous samples (Aim 1a). With an eye toward improving theory and the intervention, the proposed research will also identify students who benefit the most (or least) from the incremental theory message (Aim 1b). Finally, daily-diary (Aim 2) and laboratory stress (Aim 3) methods will identify the principle mediating social-cognitive, neuroendocrine, and cardiovascular reactions that are implicated in the treatment effects. All hypotheses have received support in preliminary data. The incremental theory intervention has reduced cortisol over a week and immediate cardiovascular threat responses to social evaluative stressors in the laboratory. In summary, this research aims to use large samples to provide needed evidence about the use of an incremental theory intervention to prevent internalizing symptoms that accompany the transition to high school, and uncover social-cognitive and biological mechanisms.